The goal of this Core is to design, synthesize and evaluate selected chemical modifications to small interfering[unreadable] RNAs (siRNAs) for improved chemical stability and delivery to cells within the cervicovaginal mucosa that[unreadable] are important in HIV transmission. Previous experience with in vivo applications of oligonucleotides[unreadable] (including antigene, antisense, decoy, and ribozyme therapies) has clearly demonstrated the need for[unreadable] chemical modification to improve pharmacokinetics in mammalian systems. Although there are virtually no[unreadable] technical limitations regarding chemical modification of the nucleobase, sugar and phosphate moieties of[unreadable] RNA, interactions of the siRNA with the proteins involved in gene silencing puts constraints on the types of[unreadable] modifications and the number of residues that can be modified without severely impairing the efficacy of the[unreadable] siRNA. Optimization of delivery, stability, and activity of siRNAs for vaginal delivery will require an[unreadable] assessment of a range of chemical modifications for incorporation within and near the termini of siRNAs in[unreadable] combination with cholesterol or CCR5-ligand conjugation for enhancement of uptake into cells susceptible to[unreadable] infection. Finally, the selected siRNAs will be appropriately formulated for clinical intravaginal application.[unreadable] This Core will provide modified and formulated siRNAs to each of the Projects for in vitro and in vivo assays[unreadable] to test the potential of specific modifications for gene silencing and for protection against viral transmission in[unreadable] cell lines, primary cells and cervicovaginal explants (Project by Lieberman), in mice (Project by Palliser) and in rhesus macaques[unreadable] (Project by Veazey). The ultimate commercial product from this study will be a drug that can be used vaginally to[unreadable] prevent infection by HIV. In particular, our aims are to (1) improve biological stability of siRNA duplexes by[unreadable] adding phosphorothioate linkages to the backbone and by modification of the 2' position of the sugar[unreadable] residues; (2) improve cell targeting and permeation by conjugation of cholesterol, CCR5-specific ligands, or[unreadable] mannose to siRNA; and (3) develop an siRNA formulation suitable for clinical intravaginal application.